The present invention relates to an ultrasonic motor which is provided with a detecting polarized portion for detecting a drive signal based on oscillation in a driving polarized portion, and more particularly to an ultrasonic motor and to an electronic apparatus or appliance having the ultrasonic motor having a detecting polarized portion to detect the drive signal with a drive frequency component that is great relative to unwanted frequency components.
Recently, attention has been drawn to ultrasonic motors utilizing a piezoelectric-element having piezoelectric effects in the field of the micro-motors.
An ultrasonic motor of this type is applied with an alternating current obtained generally in an external oscillation circuit, which adopts a method of detecting a phase difference in electric current or voltage in order to trace a resonant point that varies in response to change in the external environment, including temperature and load. This method, however, renders the circuit complicated. Under such a situation, self-oscillation circuits have been placed into practical application by utilizing self-oscillation caused by a piezoelectric vibrating member itself having a piezoelectric element. The self-oscillation circuits have a feature that the structure is simple without requiring a tracing circuit. The self-oscillation circuits include a method for causing oscillation by feeding a drive signal obtained through a detecting electrode separately provided from a drive electrode back to a driving electrode through an amplifying circuit, and a method for oscillation utilizing the inductiveness at a resonant point of a piezoelectric element.
FIG. 33 shows a first conventional example of an ultrasonic motor provided with a self-oscillation circuit.
This ultrasonic motor is provided with an annular piezoelectric vibrating member 101 formed by overlaying an elastic member and a piezoelectric element processed with a predetermined polarization, a first polarized portion 102a and first electrode group 102b circumferentially formed on a surface of a piezoelectric element along the piezoelectric vibrating member 101 to have a length corresponding to a 1/2 wavelength of a circumferential standing wave caused through electrodes, a second polarized portion 103a and second electrode group 103b provided at a position deviated by a 1/4 wavelength from the first polarized portion 102a and the first electrode group 102b, respectively, a third electrode 104 provided between the second polarized portion 103a, second electrode group 103b and the first polarized portion 102a, the first electrode group 102b having a circumferential length corresponding to a 1/4 wavelength, a detecting polarized portion 105a and detecting electrode 105b positioned opposite to the third electrode 104 to have a circumferential length corresponding to a 3/4 wavelength, a first power amplifier 106 having an input end connected to the second electrode group 103b, a band pass filter amplifier 107 having an input end connected to the detecting electrode 105b and an output end connected to the first power amplifier 106, a π/2 phase shifter 108 having an input end connected to an output end of the band pass filter amplifier 107, and a second power amplifier 109 having an input end connected to an output end of the π/2 phase shifter 108 and an output end connected to the first electrode group 102b (see JP-B-6-01191).
According to the foregoing conventional ultrasonic motor, the drive signal detected through the detecting electrode 105 is extracted by the band pass filter 107 of only a principal resonant frequency component which is amplified by the first power amplifier 106 and then fed back to the second electrode group 103b. The resonant frequency component, on the other hand, is inputted to the π/2 phase shifter 108 where the phase is shifted by π/2, and then amplified by the second power amplifier 109, being fed back to the first electrode group 102b. On the piezoelectric vibrating member 101 are caused a first standing wave due to oscillation in the first polarized portion 102a and a second standing wave deviated in phase by π/2 due to oscillation in the second polarized portion 103a. Thus a traveling wave is caused in the circumferential direction of the piezoelectric vibrating member 101, thereby obtaining a drive power.
FIG. 34 shows a second conventional example of an ultrasonic motor provided with a self-oscillation circuit.
This ultrasonic motor is provided with a piezoelectric vibrating member 111 similarly to the first conventional example, a first polarized portion 112a and first electrode group 112b, a second polarized portion 113a and second electrode group 113b, a first detecting polarized portion 114a and first detecting electrode 114b formed in one of gaps between the first electrode group 112b and second electrode group 113b, a second polarized portion 116a and second detecting electrode 116b provided in the other gap 115 between the first electrode group 112b and the second electrode group 113b and adjacent to the first electrode group 112b, a third polarized portion 117a and third detecting electrode 117b provided adjacent to the second electrode group 113b, a first self-oscillation portion 119a having an input end connected to the first detecting electrode 114b and an output end connected to the first electrode group 112b, and a second self-oscillation portion 119b having an input end connected to the second detecting electrode 116b or third detecting electrode 117b through a change-over switch 118 and an output end connected to the second electrode group 113b. 
According to the foregoing conventional ultrasonic motor, a signal with a π/2 or −π/2 phase deviation is detected by the first detecting electrode 114b and second detecting electrode 116b or the first detecting electrode 114b and third detecting electrode 117b. Consequently, there is no necessity of using a π/2 phase shifter as required by the first conventional ultrasonic motor (see JP-A-8-317672).
In the first conventional art, however, the detecting polarized portion 105a and the detecting electrode 105b are provided asymmetrical with respect to a loop of a first flexion vibration wave caused in the piezoelectric vibrating member 101 through the first electrode group 102b. This is also true for the positional relationship of a second flexion vibrating wave caused in the piezoelectric vibrating member 101 through the second electrode group 103b. If in this manner the detecting polarized portion 105a and the detecting electrode 105b are arranged in an asymmetrical relationship with respect to a loop of the flexion vibrating wave, the detecting polarized portion 105a deforms in an asymmetrical fashion. The drive signal detected through the detecting electrode 105b contains unwanted frequency components due to spurious vibrations, besides a drive-mode frequency component. The drive-mode frequency component signal obtained is decreased, resulting in instability of self-oscillation.
Meanwhile, because the piezoelectric vibrating member 101 is oscillated by two signals that are different in phase, the phase shift circuit 108 has an increased load and, on the other hand, the gain of self-oscillation loop lowers. This results in instability of oscillation or impossibility of oscillation as the case may be.
In the second conventional ultrasonic motor, on the other hand, the first detecting polarized portion 114a and the first detecting electrode 114b are arranged intermediate between a node and a loop of a first flexion vibrating wave caused in the piezoelectric vibrating member 111 through the first polarized portion 112a. Further, the second detecting electrode 116b and the third detecting electrode 117b are also provided intermediate between a node and loop of a second flexion vibrating wave caused in the piezoelectric vibrating member 111 through the second polarized portion 113a. Due to this, spurious oscillation is also liable to occur wherein there is a tendency of decreasing a desired frequency component, resulting in instability of self-oscillation.
Meanwhile, where the detecting polarized portion and the detecting electrode are provided in a part of the driving polarized portion, if the detecting polarized portion and detecting electrode are decreased in area, it is generally impossible to detect a drive signal great in magnitude. Conversely, if the detecting polarized portion and detecting electrode are made broad in area, the driving polarized portion is decreased in area, resulting in a problem of incurring a decrease in the drive force.